This application contains a numbered list of references that contains helpful material for an understanding of the invention and to further support the following specification. Throughout this specification, where appropriate, reference is made to these references by their assigned number.
Various types of optical guided mode switches are needed in multimedia networks [1-3]. These switches must be capable of handling and processing optical guided modes as in optical communication systems, guided modes are almost exclusively dealt with. Accordingly, guided mode switches must be used where their development is a challenging task and requires a much greater effort compared to that needed in development of unguided light beam switches. As examples, optical ATM (Asynchronous Transfer Mode) switches are introduced in multimedia networks where 1 out of N inputs is wavelength routed to a required output via a switch matrix [4]. Optical cross connects are required to replace electronic SDH (Synchronous Digital Hierarchy) to surpass speed limitations of electronic circuitry and allow for wavelength multiplexing in high capacity networks [5]. In such cross connects, spatial switch matrices are usually used.
Fast switching operation is achievable through electro-optic activity where refractive index is changed by electric field application to an electro-optic material. For instance, Lithium Niobate largely responds to such field application when the z-cut orientation is chosen to use the electro-optic coefficient r33 of the crystal. System applications using Lithium Niobate switch matrices have actively been pursued. Y branch structures have been adopted in realization of Lithium Niobate 1×2 switches and experimental verification of switching action over a 300 nm wavelength range has been carried out [6,7].
In semiconductor switches, refractive index change is based on QCSE (Quantum Confined Stark Effect) or current injection to create free carrier plasma effect [8, 9, 10, 11]. Optical amplifiers have been used to compensate for losses in highly absorbent semiconductors [12]. Slowing light in Bragg reflectors where effective refractive index drastically increases has also been used in total internal reflection switches [14].
Polymers were also used in realization of Digital Optical Switches (DOS) where electrooptic and passive polymers have been integrated on the same chip [15]. This same effect has also been used in silica based PLC (Planar Lightwave Circuits) where light phase of Mach Zehnder interferometers is changed to achieve switching operation [16, 17].
In all the above-mentioned examples, overall performance of the switch matrix is dominantly determined by the switch element. That is, the switch element must exhibit properties that include low-loss, high extinction-ratio, low cross-talk, low drive voltage, polarization-independence, small size, reliability and capability of switching at speeds required for specified applications. Depending on requirements in regards to system applications of spatial switches, proper choice of switch material and physical phenomenon upon which the switching mechanism is to be based, are very important, but not easy tasks. For example, when switches are to be used in high speed multimedia networks, mechanical, thermo-optic or even current injection switches are not suitable candidates. Where inherently low loss optical switch elements are preferred, highly absorbent semiconductors may only be considered in association with optical amplifiers. When optical switch size needs to be small on the order of several hundred micrometers, Lithium Niobate which exhibits Pockels electro-optic activity, is not a good material.
Numerous research activities have been carried out to overcome these difficulties. For example, new opto-ceramic materials such as PLZT (Polycrystalline Lanthanum-modified lead Zirconate Titanate) have been developed that are capable of exhibiting refractive index changes much larger than that of Lithium Niobate [18, 19]. However, further investigation will be required to determine their suitability with respect to application in fast switches.
The instant invention provides a switch element using natural or artificial electro-optic materials which can be used as a building block of switch matrices. It can operate because of a relative refractive index change of only 0.0001˜0.0002 (see Equation (1) for the definition). To the best of Applicant's knowledge, this is the smallest required refractive index change ever achieved in a fast spatial switch. Therefore, even Lithium Niobate that shows weak but fast electro-optic activity, can be used in realization of such switches. The basis of operation for these switches is a voltage-induced refractive index change in materials that exhibit Pockels or Kerr electro-optic effects, these effects being fast phenomena. Applicant's switches can, therefore, operate at speeds in excess of 100 GHz and are capable of replacing prior art switches based on thermo-optic or plasma type current injection. These prior art switches are incapable of responding to such high speeds. Losses in Applicant's switch element are shown to be acceptably small requiring no optical amplification unless in very largely integrated switch matrices. In Applicant's switch, both extinction ratios of reflection and transmission ports, are very large (>45 dB) due to Applicant's proposed REABEL (Rear Edge Adjusted Broken ELectrode) configuration and because of the introduction of a second blocker electrode. These features cause cross-talk to be better than 50 dB even under severe conditions. In cases that other materials, such as quantum-well semiconductors, opto-ceramics, etc. are used to construct Applicant's switch, refractive index change as a response to an applied voltage is larger than that in Lithium Niobate or Polymers. The size of the switch matrix may, therefore, be greatly reduced and application into optical networks becomes eased. Integration of Applicant's switch elements in matrices of large dimensions is greatly facilitated due to the spatial perpendicularity of the switching ports, which is achieved because of the introduction of an air groove into the switch configuration. Such air grooves reflect an incident guided optical mode, or in another embodiment, reflect the guided mode in both ports, to make these ports generally perpendicular or in opposition. That is, following successful separation of the optical fields corresponding to the two switch states, the guided mode is made to undertake a loss-free total reflection into the transmission port output.